Transmission circuit



Dec. 3, 1940. R MERTZ 2,223,736

TRANSMISSION CIRCUIT Filed March 24, 1938 lA/VENTOR F. MERTZ ATTORNEYPatented Dec. 3, 1940 UNITED STATES PATET TRAN SMIS SION CIRCUITApplication March 24, 1938, Serial No. 197,772

4 Claims.

The present invention relates to transmission circuits and moreparticularly to means for coupling circuits having differentcharacteristic impedances.

In general, a signal supply circuit may becoupled to a line or receivingcircuit of radically different impedance value in two ways, first, bythe use of a resistance pad, and second, by the use of a transformer.

While the resistance pad is effective for a signal band includingcurrent variations of zero frequency and extending to an upper limitingfrequency of a million or more cycles per second, if the elements areproperly constructed, it introduces a transmission loss which is large,for example, of the order of 30 decibels when matching an output orsupply circuit of several thousand ohms with a line or receiving circuithaving an impedance of the order-of ohms or when matching a 100-ohmsupply circuit with a receiving circuit having an impedance of severalthousand ohms. On the other hand, a transformer introduces Very littletransmission loss in the frequency range over which it is effective.However, no transformers have so far been designedwhich can effectivelytransfer a signal including current variations extending over theextremely wide frequency range mentioned above.

It has heretofore been proposed to provide a coupling for two lines, theratio of the impedances of which varies with frequency, by means of aunit which is adapted to provide an impedance transformation having avariable ratio over, a wide band of frequencies. The purpose of the unitis to substantially eliminate all reflections throughout the band, andthe coupling is effected in such manner that the circuit acts at lowfrequencies as if the line were directly connected without any shuntingdevices.

Nyquist Patent 1,959,494, issued May 22, 1934, discloses means forcoupling input and output circuits, comprising a network" including aplurality of transformers which supplement each other, each serving fora part of the current in a wide frequency range, 'the network operatingas a whole to transform, with, uniform attenuation, signal currentvariations extending over a wider range of frequency than would bepossible for any one of the component transformers.

When terminal apparatus is coupled to a line, the characteristicimpedance of which varies rather widely throughout the frequencyband tobe transferred, the question arises whether the terminating impedanceviewed by the line should be its own characteristic impedanceor whetherit should be a pure resistance. It is found that in general the latteris advantageous, if the resistance is made equal to the asymptoticcharacteristic impedance of the line, i. e., the resistance approachedby the characteristic impedance of the line for the higher frequencies.The advantages are, first, the greater simplicity of the termination,and secondly, the fact that the impedance mismatch in this case producesa measwe of equalization in both the attenuation and 10 delay,particularly the latter, of the over-all connection. I

This invention is directed to a coupling unit of the type disclosed inthe Nyquist patent, but which operates to effect the transfer, from onecircuit to another, of a signal comprising a band of current variationswhich may extend down to approximately a zero frequency component or mayinclude direct current.

An object of the present invention is: to provide for transformingcurrents of wide frequency range, extending down to and including directcurrent, from one voltage to another with an attenuation which, thoughnot uniform, permits actual transfer of useful vsignal throughout thewhole range, and with a constant ratio of transformation, so that a pureresistance of suitable value on the high impedance side is transformedinto a pure resistance of lower value on the low 3 impedance side.

A feature of the invention relates to a unit, which has a substantiallyconstant impedance transformation ratio for current variations extendingover a wide frequency range including direct current, for coupling twocircuits having respectively different impedance characteristics.

By way of example, the invention will be hereinafter described as,applied to a system in which the terminal amplifier of a televisionequipment, adapted to produce a television signal having currentvariations extending from one or more million cycles down tosubstantially, and even including, zero frequency, is coupled to acoaxial conductor by a network, which comprises an autotransformerhaving its two windings and two resistances, each shunted by a by-passcondenser, connected in series with each other, and with the seriescircuit connected to the output terminals of the amplifier, and theinput terminals of the coaxial conductor connected to the network at 50points such that one winding of the transformer and one of theresistances, with its associated bypass condenser, are connected inseries with each other across the conductor terminals.

A detail description of the invention follows and is illustrated in theattached drawing, in which the single figure shows a coupling circuitembodying the invention.

Referring to the drawing, an image current, having an upper limitingfrequency of a million or more cycles and direct current as a lowlimiting frequency, after being amplified by a thermionic dischargedevice I, or other suitable amplifier, is supplied to a series circuit 2comprising a resistance 3 shunted by a condenser 4, an inductance 5, aresistance 6 shunted by a condenser 1, and an inductance 8, which ismutually coupled to the inductance 5 to constitute an autotransformer.

A coaxial conductor, comprising an outer tubular conductor 9 enclosing aconductive wire I0, is connected at points II and I2, respectively,

i. e., across the portion of the series circuit 2,

which includes the resistance-condenser combination 6! and theinductance 8. 7

By selecting the values of the resistances 3 and 6, the capacities ofthe condensers 4 and 1, respectively in shunt thereto, and providing anautotransformer of the proper design, a coupling network is providedwhich terminates the cable in approximately a' pure resistance at allfrequencies within the signal band, the value of such resistance beingsubstantially equal tothe asymptotic characteristic impedance of theline, i. e., the resistance approached by the characteristic impedanceof the line at the high frequency end of the signal band to betransmitted.

The values of the condensers 4 and I should be so chosen that theyprovide low impedance paths in shunt to the resistances 3 and 6,respectively, for current variations in the upper portion of thefrequency range of the signal band, whereby these highfrequency'variations are bypassed about these resistances and areapplied to the autotransformer, which operates to transfer them from theamplifier to the line.

Since the condensers are ineffective as coupling units and thetransformers are ineffective at zero and low frequencies, currentvariations in the lower portion of the frequency range of the signalband are transferred by means of the resistance pad comprising theresistances 3 and 6. This resistance unit causes more loss than thetransformer and, in fact, serves to introduce a transmission loss whichis high at the low frequency end of the range. However, this occurs in aportion of the frequency range where such loss is of relatively minorimportance and may even be beneficial in that the resistance unit actsin part as an equalizer to compensate for the low attenuation of thecable.

A network of the type described above may be designedso that theautotransformer operates alone to transfer current variations havingfrequencies lying in a range extending from a relatively low frequencyto a million, or even several million, cycles per second, from an outputor supply circuit having a substantially constant impedance of severalthousand ohms, to a coaxial conductor circuit having an impedance of theorder of ohms, while for current variations lying in a frequency rangeextending from approximately the lower limiting frequency transferred bythe transformer, down to and including zero frequency, or directcurrent, the resistance unit alone serves to effect the transfer. In alimited frequency range, intermediate that transferred by thetransformer and that transmitted by the resistance, both of thesecoupling units will cooperate to effect energy transfer.

As described above, the network comprises a transducer having thecomplete series circuit included between its input terminals and aportion only of the series circuit connected to its output terminals.However, the invention contemplates the use of the transducer in thereverse manner,

i. e., it may be used to couple a low impedance input or supply circuitto a high impedance output or receiving circuit. In this case, thesupply circuit is connected to the intermediate terminals, i. e., thosehaving the resistance-condenser combination 6'! in series with thewinding 8 connected between them, while the receiving circuit isconnected to the outer terminals, i. e., those having the completeseries circuit connected between them.

According to this invention, a unit which may be used to couple aninput, or signal supply circuit, having an impedance of the order of10,000 ohms to a receiving circuit having an imepdance of the order of100 ohms, or in the reverse manner, may, for example, comprise aresistance 3 of 10,000 ohms shunted by a condenser 4 of. 655.

mmf., a winding 5 and a winding 8, the combined self-inductance of thesetwo windings being of the order of 50 millihenries and that of winding.

8 being approximately .5of a millihenry, a resistance 6 of 100 ohmsshunted by a condenser l of .0655 of a microfarad, and mutual inductanceof 5 millihenries between the windings 5 and 8.

Such a network will operate to transmit a cur-. rent band including zerofrequency andextend ing to an upper limiting frequency of one or more.million cycles per second, thejexact value of the;

upper limiting frequency will depend largely upon the construction ofthe autotransformer and may be extended to several million cycles. 4

The transition from resistive to inductive coupling occurs atapproximately 16 kilocycles, i. e.,

at this frequency both couplings operate to effect the transfer ofapproximately equal amounts of the signal energy from one circuit totheother.

For a limited region in the neighborhood of 16, kilocycles, signalenergy is transferred in varying amount by the respective couplings,depending upon whether the energy to be'transferred lies between 16kilocycles and the lower limiting frequency of this region or in therange above 16 kilocycles, and, outside this region, energy transfer iseffected solely by one coupling or the other.

' While the invention has been described herein. as applied totelevision transmission, and specific. details have been given. for aparticular case, it is to be understood that the coupling unit hereindisclosed may be used in other types of signalsystems, in which signals,extending over a wide frequency band that includes direct current, aretobe transferred with constant transformation ratio between circuitshaving radicallydifferent:

impedance values in the frequency range of the signals to betransmitted.

What is claimed is:

a wide frequency range including a zero freconnected in shunt with thefirst resistor, a second condenser connected in shunt with said sec-{ond resistor, and a transformer having 'a first winding and a secondwinding, one terminal of said first resistor and one terminalof saidsecond resistor being connected by a single current path comprising saidsecond winding, said first winding and said first resistor beingconnected in series with each other between one set of terminals, andsaid second winding and said first and second resistors being connectedin series with each other across the other set of terminals.

2. A transducer having input and output terminals for coupling a sourceof signals having a wide frequency range including zero frequency or anapproximately zero frequency component to a load to obtain a low loss athigh frequencies and to maintain a constant resistance termination tothe load, comprising a first resistor and a second resistor, a firstcondenser connected in shunt with the first resistor, a second condenserconnected in shunt with said second resistor, a transformer having afirst winding and a second winding, one terminal of said first resistorand one terminal of said second resistor being connected by a singlecurrent path comprising said second winding, said first winding and saidfirst resistor being connected in series with each other between one setof terminals, and said second winding and said first and secondresistors being connected in series with each other across the other setof terminals, and a third resistor connected across said other set ofterminals.

3. A transducer having input and output terminals for coupling a sourceof signals having a wide frequency range including zero frequency or anapproximately zero frequency component to a load to obtain a low loss athigh frequencies and to maintain a constant resistance termination tothe load, comprising a first resistor and a second resistor, a firstcondenser connected in shunt with the first resistor, a second condenserconnected in shunt with said second resistor, a transformer having afirst winding and a second winding, one terminal of said first resistorand one tenninal of said second resistor being connected by a singlecurrent path comprising said first and second windings in series,

said first winding and said first resistor being connected in serieswith each other between one set of terminals, and said first and secondwindings and said first and second resistors being connected in serieswith each other across the other set of terminals, and a third resistorconnected across said other set of terminals, the resistance of saidfirst resistor being less than that of said second resistor, theself-inductance of said first winding being less than that of saidsecond winding and the capacitance of said first condenser being greaterthan that of said second condenser.

4. A coupling arrangement for connecting a I source of high andsubstantially constant impedance over a range of frequencies extendingupward from substantially zero to a load having a characteristicimpedance which varies widely for frequencies within said range, whichcomprises a series circuit including in series said source and anelement made up of a resistance and a capacity in parallel with eachother and a second element consisting of a transformer coil carrying allof the current of said series circuit at the lower frequencies of saidrange and a third element in said series circuit made up of a resistanceand a capacity in parallel with each other, said transformer beingbetween said two parallel arrangements, and a load having its terminalsconnected respectively to a midtap of said coil and to a terminal of oneof said parallel arrangements remote from the terminal thereof connectedto said transformer, whereby said condensers :are ineffective ascoupling elements and said transformer is effective as a couplingelement at high frequencies but is ineffective as a coupling element atzero and low frequencies and current variations for zero and lowfrequencies are transferred by means of said resistances which introducerelatively high transmission loss at the lower end of said frequencyrange.

PIERRE MERTZ.

